The present invention relates to a novel microwave-assisted process for the preparation of ceramic products, preferably large-sized, thick-walled ceramic products. The process is particularly suited for the preparation of large sized, thick-walled ceramic containers for the transport, storage, long-term containment and environmental isolation of nuclear and other hazardous waste.
There are currently 109 nuclear reactors in operation in the United States alone, with a further 424 worldwide. The average nuclear power plant produces about 20 metric tons of spent fuel each year. Current estimates are that the United States already has 28,000 metric tons of spent fuel stored. This number is expected to rise to 48,000 metric tons by 2003 and to 87,000 metric tons by 2030. These numbers do not reflect any waste from military nuclear applications. The safe and cost-effective disposal of this nuclear waste has created critical problems for the nuclear energy industry, for governments world wide and for the life, health and safety of humanity.
The process of the invention can produce ceramic containers that overcome the technical limitations and high cost of metal dry cask encapsulation or using glass or ceramic immobilization techniques, which are currently used.
In addition to ceramic containers for nuclear waste, the process of the invention is also applicable for producing any large size, thick-walled product that may be desired. For example, it may be used to produce high voltage electrical insulators, food and chemical apparatuses, engine blocks, very large slabs of ceramics that may be used to provide an exterior curtain wall on buildings; or roof tiles, large tiles for interior walls and floors; or structures either in one piece or a minimum number of pieces that combine floor, walls and ceiling for bathrooms, kitchens and other sanitary facilities. With large-sized ceramic plates that can be produced by the process of the invention, it is possible to substantially reduce the amount of skilled masonry work and tilesetting labor required to cover surfaces. This is desirable also in view of the declining skills of tilesetters. With large-sized plates, it is possible to minimize the number of joints that require grouting. With less grouting, there is less need for scrupulous cleaning to avoid mildew build-up over time and use, in the porous grout between the ceramic tiles. This is of particular benefit in hospitals--particularly in operating rooms--and other areas that must be maintained in a microbe-free state--such as food processing and pharmaceutical plants. The process of the invention is also useful in producing at less cost conventional ceramic products, such as sanitary ware.
Thousands of years ago man developed a ceramic technology for the conversion of a clay mass into stone-like shaped products with thin walls and moderate size at most.
Ceramic sewage and drainage pipes have been produced from ordinary clay for about two hundred years. Those pipes have a length up to 2,500 mm. Sewage pipes also may have an inner diameter up to 1,100 mm as described in ASTM, Designation: C 1208M-94, "Standard Specification for Vitrified Clay Pipe and Joints for Use in Jacking, Sliplining, and in Tunnels".
Vitrified clay pipes should be non-permeable for water solutions and be resistant and reliable for use in all possible combinations of aggressive organic and non-organic chemical hazards. However, these pipes are manufactured by conventional firing from conventional fire clay, shale, surface clay, or a combination of these materials. The firing of such large ceramic products requires long firing times with slow ramp up and down of temperature and considerable cost in conventional energy forms and consequential adverse effects on the environment.
Ceramics have low thermal conductivity. Therefore, the speed of conventional ceramic processing or an increase in the temperature of the external heating is inversely related to the body volume of the ceramic product. The greater the body volume, the lower the rise in temperature per unit of time and the longer the firing process. Consequently there is lower energy efficiency and, correspondingly, higher processing cost. The use of conventional heating methods in the production of large ceramic items requires a considerable amount of time and money to decrease body thermal gradients during a very long firing process.
The theory of microwave ceramic processing has not been fully developed. Some patents have issued on the use of microwaves in ceramic processing, however. Illustrative of such patents may be mentioned the following:
U.S. Pat. No. 3,585,258, granted Jun. 15, 1971, describes a method of firing a ceramic article by microwave energy at a firing temperature higher than available from the exposure of the article to the microwave energy which comprises locating the article in such a position as to subject it to the microwave energy, locating a plurality of loose or divided particles of a lossy material selected from the group consisting essentially of ferrites, iron, iron ore, and carbon in a predetermined fixed relationship with the article, which on exposure to microwave energy, provides the required firing temperature to fire the article, emitting microwave energy to the lossy material to generate a multitude of arcs therethrough, thereby generating high refractory heat energy for firing the article.
U.S. Pat. No. 4,147,911, granted Apr. 3, 1979, describes a method for sintering refractories which comprises mixing a group of dielectric refractory particles with 0.1 to 5% by weight of an electric conductor, in which the particle size of the electric conductor is not more than 10 times the skin depth of the conductor in the microwave region, forming the mixture and sintering the formed mixture thus obtained by means of microwave induction heating in a space surrounded by a metal wall.
In U.S. Pat. No. 4,687,895, granted Aug. 18, 1987, a system for heating objects serially within a plurality of separate heating zones is described. Objects are placed on pallets and sequentially moved through the plurality of heating zones. In a preferred embodiment, the pallets are fabricated of a microwave absorptive material and sequentially moved through a plurality of microwave cavities. The microwave cavities are provided with a source of microwave energy which heats the pallets, and by conduction, the objects placed thereon.
U.S. Pat. No. 4,771,153, granted Sep. 13, 1988, describes an apparatus for heating a ceramic by microwave power. The apparatus has a cavity resonator in which the ceramic is placed. The resonator is provided with a variable iris. The apparatus detects the temperature of the ceramic or other state of the ceramic, and adjusts the area of the opening in the iris in the resonator and the resonant frequency of the resonator according to a signal produced by the detection, in order to bring the resonator substantially into resonance and the degree of coupling to exactly or nearly unity. Alternatively, the apparatus adjusts its microwave power for these purposes.
U.S. Pat. No. 4,880,578, granted Nov. 14, 1989, discloses a method for microwave sintering of materials, primarily metal oxides. Metal oxides that do not normally absorb microwave radiation at temperatures ranging from about room temperature to several hundred degrees centigrade are sintered with microwave radiation without the use of the heretofore required sintering aids. This sintering is achieved by enclosing a compact of the oxide material in a housing or capsule formed of an oxide which has microwave coupling properties at room temperature up to at least the microwave coupling temperature of the oxide material forming the compact. The heating of the housing effects the initial heating of the oxide material forming the compact by heat transference and then functions as a thermal insulator for the encased oxide material after the oxide material reaches a sufficient temperature to adequately absorb or couple with microwave radiation for heating thereof to sintering temperature.
In U.S. Pat. No. 4,963,709, granted Oct. 16, 1990, a microwave sintering system and method are provided for sintering of large and/or irregular shaped ceramic articles at microwave frequencies of at least 28 GHz in the hundreds of kilowatts power range in an untuned cavity. A 28 GHz, 200 kW gyrotron with variable power output is used as the microwave source connected to an untuned microwave cavity formed of an electrically conductive housing through an overmoded waveguide arrangement which acts in conjunction with a mode promoter within the cavity to achieve unexpected field uniformity. The part to be sintered is placed in the cavity and supported on a removable high temperature table in a central location within the cavity. The part is surrounded by a microwave transparent bulk insulating material to reduce thermal heat loss at the part surfaces and maintain more uniform temperature. The cavity may be operated at a high vacuum to aid in preventing arcing. The system reportedly allows controlled increased heating rates to provide rapid heating of a ceramic part to a selected sintering temperature where it is maintained by regulating the microwave power applied to the part.
U.S. Pat. No. 5,010,220, granted Apr. 23, 1991, discloses a process and apparatus for heating bodies to high temperatures at high pressures. The process involves locating the body in a chamber capable of acting as a resonant cavity for microwave radiation of a predetermined frequency. The body is then irradiated in the cavity with microwave energy of the predetermined frequency for a time sufficient to raise the temperature of the body to a suitable high temperature. Then, either subsequently or simultaneously, a fluid at high pressure is introduced into the cavity to pressurize the body. The process can be used for sintering and/or hot isotactic pressing of bodies made of ceramic powder and for similar purposes.
U.S. Pat. No. 5,072,087, granted Dec. 10, 1991, describes a process for preparing a heat-treated body from a material (preferably a dielectric ceramic) that does not couple well with microwaves while nevertheless using microwave energy for the heating step. The process involves the use of a microwave susceptor (i.e., a material that couples well with microwaves) as a means for generating heat in the material. To avoid contamination of the final product, a susceptor is chosen which is converted, during the heating step, to a substance which is substantially the same as the material itself, both the susceptor and the material are converted to the same desired final product, or the material is converted to a substance substantially the same as the susceptor. The resulting substantially heat-treated (and preferably sintered) bodies can be used for a variety of purposes, e.g., as substrates for micro-electronic devices. The process can also be used for joining bodies of nonsusceptor materials.
In U.S. Pat. No. 5,164,130, granted Nov. 17, 1992, a method for sintering ceramic materials is described. A ceramic article is coated with layers of protective coatings such as boron nitride, graphite foil, and niobium. The coated ceramic article is embedded in a container containing refractory metal oxide granules and placed within a microwave oven. The ceramic article is heated by microwave energy to a temperature sufficient to sinter the ceramic article to form a densifed ceramic article having a density equal to or greater than 90% of theoretical density.
U.S. Pat. No. 5,202,541, granted Apr. 13, 1993, discloses a method of heating a workpiece assembly and a load assembly suitable for heating by the method. The method involves heating the workpiece assembly in a microwave cavity surrounded by one or more rings made of electrically conductive material. The rings fix the electrical field in such a way that uniform heating of the workpiece assembly can be achieved. Large workpieces or assemblies can be heated and, if sinterable, sintered in this way without the problems normally caused by lack of uniform fields when microwaves are used to heat large loads.
U.S. Pat. No. 5,223,186, granted Jun. 29, 1993, describes a method of sintering nanocrystalline material wherein the nanocrystalline material is microwaved to heat the material to a temperature less than about 70% of the melting point of the nanocrystalline material expressed in degrees K. This method produces sintered nanocrystalline material having a density greater than about 95% of theoretical and an average grain size not more than about 3 times the average grain size of the nanocrystalline material before sintering. Rutile TiO.sub.2 as well as various other ceramics have been prepared. Grain growth of as little as 1.67 times has resulted with densities of about 90% of theoretical.
U.S. Pat. No. 5,321,223, granted Jun. 14, 1994, discloses a method of sintering a material with microwave radiation that comprises coating with microwave-absorbent carbon a compacted article comprising inorganic particles having poor microwave coupling characteristics at room temperature. The microwave-absorbent carbon does absorb microwaves at room temperature. Thereafter, the compacted article is irradiated with microwave radiation to heat the compacted article by microwave absorption of the microwave-absorbent carbon coated on the compacted article to a temperature sufficient for the inorganic particles in the compacted article to absorb the microwave radiation and for a period of time sufficient to sinter the compacted article and to remove the microwave-absorbent carbon coated on the compacted article.
U.S. Pat. No. 5,365,042 granted Nov. 15, 1994 describes a heat treatment installation for parts made of a composite material having a ceramic matrix and including a treatment enclosure which is connected to a microwave generator by a waveguide and which includes a press for hot pressing a part to be treated in the enclosure and a gas source for introducing a protective gas into the enclosure.
U.S. Pat. No. 5,521,360, granted May 28, 1996, discloses a variable frequency microwave heating apparatus designed to allow modulation of the frequency of the microwaves introduced into a furnace cavity for testing or other selected applications. The variable frequency heating apparatus is used in a method to monitor the resonant processing frequency within the furnace cavity depending upon the material, including the state thereof, from which the workpiece is fabricated. The variable frequency microwave heating apparatus includes a microwave signal generator and a high-power microwave amplifier or a microwave voltage-controlled oscillator. A power supply is provided for operation of the high-power microwave oscillator or microwave amplifier. A directional coupler is provided for detecting the direction and amplitude of signals incident upon and reflected from the microwave cavity. A first power meter is provided for measuring the power delivered to the microwave furnace. A second power meter detects the magnitude of reflected power. Reflected power is dissipated in the reflected power load. In the field of microwave radiation, it is well known that microwave furnaces are typically constructed with a fixed operating frequency. It has long been known that the interactions of various materials with microwaves are frequency dependent. These interactions may include sintering ceramics.
U.S. Pat. No. 5,432,325 granted Jul. 11, 1995 discloses an apparatus and method for high temperature sintering of plate-shaped articles of alumina, magnesia, silica, yttria, zirconia, and mixtures thereof using microwave radiation. An article is placed within a sintering structure located within a sintering container which is placed in a microwave cavity for heating. The rates at which heating and cooling take place is controlled.
Many ceramics, such as alumina, are poor absorbers of microwave energy at low temperatures. Clay and kaolin raw material are better microwave absorbers at low temperatures. When any green ceramic component approaches a critical temperature, it rapidly begins to absorb microwave energy. However, the surface, from which heat can be lost in ambiance in a microwave-only situation, rapidly becomes cooler than the center of the components. It may cause cracking and non-uniformity in material properties.
UK Patent No. 2,296,173 (EA Technology Ltd.) describes a process for microwave-assisted processing of ceramic products. The process uses conventional heat sources if necessary to elevate the temperature of the ceramic products at which microwave energy is effective in heating the ceramic products and also to compensate for the heat lost from the surfaces of the ceramic product from the volumetric heat supplied by the microwaves.